Pre-PEP: Executing modules inside packages with '-m'

Anyone playing with the CPython interpreter's new command line switch might have
noticed that it only works with top-level modules (i.e. scripts that are
directly on sys.path). If the script is inside a package, the invocation will
fail with a "Module not found" error.

This PEP defines semantics for executing modules inside packages as
scripts with the ``-m`` command line switch.

The proposed semantics are that the containing package be imported prior
to execution of the script.

Rationale
=========

Python 2.4 adds the command line switch ``-m`` to allow modules to be
located using the Python module namespace for execution as scripts.
The motivating examples were standard library modules such as ``pdb``
and ``profile``.

A number of users and developers have requested extension of the
feature to also support running modules located inside packages.
One example provided is pychecker's ``pychecker.checker`` module.
This capability was left out of the Python 2.4 implementation
because the appropriate semantics were not entirely clear.

The opinion on python-dev was that it was better to postpone the
extension to Python 2.5, and go through the PEP process to help
make sure we got it right.

Scope of this proposal
==========================

In Python 2.4, a module located using ``-m`` is executed just as if its
filename had been provided on the command line. The goal of this PEP is
to get as close as possible to making that statement also hold true for
modules inside packages.

Prior discussions suggest it should be noted that this PEP is **not**
about any of the following:
- changing the idiom for making Python modules also useful as scripts (see PEP
299[1]_).

- addressing the problem of ``-m`` not understanding zip imports or
Python's sys.metapath.

The issues listed above are considered orthogonal to the specific
feature addressed by this PEP.

Current Behaviour
=================

Before describing the new semantics, it's worth covering the existing
semantics for Python 2.4 (as they are currently defined only by the
source code).

When ``-m`` is used on the command line, it immediately terminates the
option list (like ``-c``). The argument is interpreted as the name of
a top-level Python module (i.e. one which can be found on ``sys.path``).

If the module is found, and is of type ``PY_SOURCE`` or ``PY_COMPILED``, then
the command line is effectively reinterpreted from ``python <options> -m
<module> <args>`` to ``python <options> <filename> <args>``. This includes
setting ``sys.argv[0]`` correctly (some scripts rely on this -
Python's own ``regrtest.py`` is one example).

If the module is not found, or is not of the correct type, an error
is printed.

Proposed Semantics
==================

The semantics proposed are fairly simple: if ``-m`` is used to execute
a module inside a package as a script, then the containing package is
imported before executing the module in accordance with the semantics
for a top-level module.

This is necessary due to the way Python's import machinery locates
modules inside packages. A package may modify its own __path__ variable
during initialisation. In addition, pathing may be affected by ``*.pth``
files. Accordingly, the only way for Python to reliably locate the module
is by importing the containing package and inspecting its __path__
variable.

Note that the package is *not* imported into the ``__main__`` module's
namespace. The effects of these semantics that will be visible to the
executed module are:

A reference implementation is available on SourceForge [2]_. In this
implementation , if the ``-m`` switch fails to locate the requested
module at the top level, it effectively reinterprets the command
from ``python -m <script>`` to ``python -m execmodule <script>``.
(There is one caveat: when reinterpreted in this way, ``sys.argv[0]``
may not actually contain the filename of ``execmodule``. This only
affects ``execmodule`` itself, not the requested module).

``execmodule`` is a proposed standard library module that contains a single
function (also called ``execmodule``). When invoked as a script, this
module finds and executes the module supplied as the first argument. It
adjusts ``sys.argv`` by deleting ``sys.argv[0]`` and replacing the new
``sys.argv[0]`` with the module's filename instead of its Python name.

The function ``execmodule`` is like ``execfile``, but uses the Python
module namespace to locate the script instead of the filesystem. It
has an additional optional argument ``set_argv0`` which causes the
filename of the located module to be written to ``sys.argv[0]`` before
the module is executed.

A hybrid C/Python implementation is used as the Python module is much
more flexible and extensible than the equivalent C code would be. It
also allows the ``execmodule`` function to be made available. Scripts
which execute other scripts (e.g. ``profile``, ``pdb``) have the option
to use this function to provide ``-m`` style support for identifying the
script to be executed.

The Python code for ``execmodule`` has also been posted as a
cookbook recipe for Python 2.4 [3]_.

Open Issues
===========

- choosing a name for the standard library module containing ``execmodule``.
The reference implementation uses ``execmodule``. An alternative name
proposed on python-dev is ``runpy``.

Alternatives
============

The main alternative implementation considered ignored packages'
__path__ variables, and looked only in the main package directory. A
Python script with this behaviour can be found in the discussion of the
``execmodule`` cookbook recipe [3]_.

This approach was not used as it does not meet the main goal of the
``-m`` switch - to allow the full Python namespace to be used to locate
modules for execution.

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